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Origin and Nature of Planetary Systems Larry Lebofsky and the NIRCam E/PO Team Introduction: While there is only one Solar System (the system of the star Sol), as of early April 2015, there are 1207 known extrasolar planetary systems with 1911 known planets (called extrasolar planets or exoplanets). Of these planetary systems 480 have two or more planets. In this activity, we will construct models of seven of these planetary systems and compare them with our Solar System. In addition to this, 84 planets orbit a star that is part of a binary star system and 22 planets orbit both stars of a binary system just like Tatooine in Star Wars! There are also two known planets that do not orbit a star—orphan planets. Scale Modeling: Size and distance scales in the Solar System and beyond are difficult to create without introducing misconceptions. When demonstrating phases of the Moon, for example, it is difficult to model simultaneously both the sizes of the Earth and Moon as well as their distances. The same is true for the modeling the sizes of the planets and their distances from the Sun, as well as the distances to the stars we see in the night sky and their planetary systems. Because of limited time and space, we, as a group, will model the Earth/Moon system, the relative sizes of the planets in our Solar System, the distance scale of our Solar System and several other planetary systems, and the distances to those systems. Materials: We have Play-Doh and plastic balls to model the Earth/Moon system (size and distance, respectively). We have laid out balls and beads to represent the planets of the Solar System. We have laid out macramé strings and twine to represent the solar distances of the planets of our Solar System (plus Pluto) and the distances of the seven other planetary systems listed below. The scale for these planetary system models is 1 meter = 100,000,000 kilometers, so the distance from the Sun to the Earth (149,600,000 kilometers = 1 Astronomical Unit [AU]) is 1.5 meters in this model. On this scale, the diameter of the Sun is about 1.4 cm and the Earth 0.13 mm. All of the models include scale-model-sized beads that also represent the colors of the stars, themselves. The planets are much too small to be seen on this scale. Because the distances are so great, the scale distance used for the string models for HR 8799 and Fomalhaut is 1 meter = 1,000,000,000 kilometers in order to conserve string. Note: As with any model the sizes and distances are approximate, limited by the sizes of available beads, etc. Activities: The Play-Doh has been divided in 50 equal balls. If you were to make a scale model of the relative sizes of Earth and Moon using all of these balls, how many would be needed for the Earth and how many for the Moon? To represent accurately the distances between them, how far apart would you need to put the Moon from the Earth (answers on page 3)? We now have a set of balls and beads that represent the planets (plus Pluto). On this scale, the Sun would be a little less than 5 feet in diameter (4.6 feet or 139 cm). On this scale, how far would Mercury be from the Sun (answer on page 3)? Using Table 1 below, we have created a scale model of our Solar System and seven extrasolar planetary systems. Do any of them look like our Solar System? How are they similar and different? Table 2 gives the distances of the stars from the Solar System in light-years (ly). Given that 1 light-year, the distance light travels in a year, equals 9.46 x 1012 kilometers or 63,241 AU, create a scale that would represent the distances to the seven planetary systems from the Solar System. How big, in your scale model, would the individual planetary systems be? We have included a model for another system. For the string model, we used the same scale as the previous systems, 1 meter = 100,000,000 kilometers. However, we have also enlarged the model to 1 meter = 10,000,000 kilometers so that we can show the smaller bodies to scale. What is this system (answer on page 3)? Hint: It was the first “planetary” system to be observed through a telescope. Discussion: How do these seven planetary systems compare to our Solar System? Because of the methods used to discover these systems, two have lower limits for their planet masses and no information about their radii. Why? Several other planets have upper limits on their masses. Why? Do you notice any patterns in the distances of the planets from their respective stars? Their masses? Origin and Nature of Planetary Systems 4-13-15.docx 1 Table 1 Star Sun HR 8799 (Imaged) Fomalhaut (Imaged) 55 Cancri (radial vel.) Upsilon Andromedae (radial vel.) Kepler-11 (transit) Kepler-20 (transit) Kepler-42 (transit) Unknown System (Imaged) Distance Planet (Lightyears) 0.0 Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto 129 e d c b 25.1 b 40.2 e b c f d 44.0 b c d e 2,000 b c d e f g 950 b e c f d 125 c b d ? 1 2 3 4 Dist. (AU) 0.39 0.72 1.00 1.52 5.20 9.58 19.2 30.1 39.3 14.5 27 43 68 115 0.016 0.11 0.24 0.78 5.76 0.06 0.86 2.55 5.25 0.091 0.106 0.159 0.194 0.25 0.462 0.045 0.051 0.093 0.11 0.35 0.006 0.012 0.015 0.003 0.004 0.007 0.013 Ang. Sep. Arcsec* 0.37 0.68 1.09 1.73 15 0.0013 0.0093 0.019 0.063 0.47 0.004 0.064 0.19 0.39 0.0002 0.0002 0.0004 0.0004 0.0012 0.0002 0.0003 0.0004 139. 220. 351. 617. Model Dist (m)** 0.58 1.08 1.50 2.28 7.79 14.3 28.8 45.0 58.7 21.8 41 65 102 172 Period (Days/ Years) 88.0d 224.7d 365.3d 687.0d 11.86y 29.46y 84.32y 164.79y 248.04y 49.3y 113y 225y 450y 880y Mass (E or J) *** 0.055 ME 0.82 ME 1.0 ME 0.107 ME 1.0 MJ, 317.8 ME 0.30 MJ, 95.2 ME 0.046 M, 14.5 ME 0.054 MJ, 17.1 ME 0.0022 ME 5-13 MJ 7-13 MJ 7-13 MJ 5-11 MJ 0.05−1 MJ 0.024 0.16 0.36 1.17 8.63 0.09 1.24 3.8 7.9 0.14 0.16 0.24 0.29 0.38 0.69 0.068 0.077 0.14 0.17 0.52 0.009 0.018 0.023 0.004 0.007 0.011 0.018 0.74d 14.6d 44.3d 261d 14y 4.62d 238d 3.57y 10.5y 10.3d 13.0d 22.7d 32.0d 46.7d 118.4d 3.70d 6.10d 10.9d 19.6d 77.6d 0.45d 1.21d 1.86d 1.77d 3.55d 7.16d 16.7d ~0.027 MJ, ~7.8 ME ~0.8 MJ ≥0.17 MJ ≥0.14 MJ ≥3.8 MJ (4.8 MJ?) 0.69 MJ 14.6 MJ 10.2 MJ 1.1 MJ 4.3 ME 13.5 ME 6.1 ME 8.4 ME 2.3 ME <300 ME 8.7 ME 0.39−1.67 ME 16.1 ME 0.66−3.04 ME <20.1 ME <2.1 ME <2.7 ME <0.9 ME 0.01 ME 0.008 ME 0.02 ME 0.02 ME Radius (E or J) Ecc. Inc. **** 0.38 RE 0.95 RE 1.0 RE 0.53 RE 10.9 RE, 1.0 RJ 9.0 RE, 0.83 RJ 4.0 RE, 0.37 RJ 3.9 RE, 0.36 RJ 0.18 RE 0.21 0.009 0.017 0.093 0.049 0.056 0.044 0.011 0.245 7.0° 3.4° 0.0° 1.9° 1.3° 2.5° 0.8° 1.8° 17.2° ~0.1 28° 28° 28° 66° 1.2 RJ 1.3 RJ 1.1 RJ 2 RE 1.97 RE 3.15 RE 3.43 RE 4.52 RE 2.61 RE 3.66 RE 1.91 RE 0.87 RE 3.07 RE 1.03 RE 42.75 RE 0.73 RE 0.78 RE 0.57 RE 0.28 RE 0.25 RE 0.41 RE 0.38 RE 0.17 0.016 0.09 0.000 0.025 0.013 0.24 0.27 0.005 83° 85° 53°? 30° 8° 24° <0.32 <0.28 <0.40 <0.32 <0.60 88.5° 89° 89.3° 88.8° 89.4° 89.8° 86.5° 87.5° 88.4° 88.7° 89.6° 0.004 0.009 0.001 0.007 87° 87° 87° 87° *As viewed from Earth. An arcsecond is an angular measurement equal to 1/60 of an arcminute and 1/3600 of a degree. Since the Moon is about one-half of a degree (30 arcminutes), an arcsecond is 1/1800 the width of the Moon. From 3.26 ly away, the Earth would have a maximum separation of 1 arcsecond from the Sun. **One meter = 100,000,000 km ***For radial velocity measurements, mass is related to the inclination of the orbit (as viewed from Earth); however, since the inclinations of the orbits cannot be measured with this method, the masses are minimum masses. ****For exoplanets, the inclination is defined by the orbit’s angle as viewed from Earth: 0°, pole on; 90°, edge on. For transits, the exoplanet orbits must be viewed nearly edge-on. For radial velocity measurements, an assumption of edge-on gives the minimum possible mass. Origin and Nature of Planetary Systems 4-13-15.docx 2 Table 2: Information about the stars Star Sun HR 8799 Fomalhaut 55 Cancri Upsilon Andromedae Kepler-11 Kepler-20 Kepler-42 Distance (LightYears) 0.000016 129 25.1 40.2 44.3 Model Dist. (km)* 0.0015 12,000 2,380 3,800 4,200 Mass (Relative to Sun) 1.00 1.47 1.92 0.95 1.28 Diameter (Relative to Sun) 1.00 1.34 1.84 1.15 1.28 2000 950 125 189,000 90,000 12,000 0.95 0.91 0.13 1.10 0.94 0.17 Spectral Type Temperature G2 A5 A3 K0 F8 5,778 K 7,430 K 8.590 K 5,370 K 8,070 K Luminosity (Relative to Sun) 1.0 4.9 16.6 0.63 3.6 G8 G8 M5 5,700 K 5,470 K 3,070 K ~1.1 ~0.8 0.0024 Age (Billion Years) 4.6 0.06 0.44 10 2.6 8 9 ? *One meter = 100,000,000 km Figure 1: Primary methods used to detect planets around other stars Imaging (HR 8799 and Fomalhaut) Radial Velocity (55 Cancri and Upsilon And.) Transit (Kepler-11, 20, and 42) Microlensing (not represented in activity) Sixteen exoplanets have also been discovered by pulsar and variable star timing. Answers to activity questions: 1. Given 50 equal-sized clay balls, you would need 49 to represent the Earth and 1 to represent the Moon. To represent the distance from the Earth to the Moon, you would have to put the Moon at a distance equal to 30 times the diameter of the Earth (the diameter of Earth is 3.67 times the diameter of the Moon, so the Earth-Moon distance would also be 110 times the diameter of the Moon). The Earth-Moon distance is represented by the string of plastic balls. 2. This scale is 1 meter = 1 million kilometers, so Mercury would be 58 meters away from the Sun (mean distance). You need this scale, relative to the distance scale, in order to be able to see the relative sizes of the planets. 3. Jupiter and its four Galilean moons! Origin and Nature of Planetary Systems 4-13-15.docx 3 Figure 2: Examples of exoplanet systems Model of Kepler 11 and its exoplanets (distances) Illustration of the Kepler 11 system: Wikipedia Model of Kepler 20 and its exoplanets (distances) Illustration of the Kepler 20 system: Space.com Setting up the string model of the 55 Cancri system (right) Illustration of the 55 Cancri system: Wikipedia Origin and Nature of Planetary Systems 4-13-15.docx 4